IGBTs have been made as punch-through devices (PT-IGBT) in which a DMOS type structure is formed at the top of an epitaxially deposited silicon substrate. The epitaxial layer is formed atop a higher concentration buffer layer, of the same conductivity type, which is in turn formed on a substrate of opposite conductivity type and which acts as a minority carrier injection source. In such punch-through devices, the electric field across the silicon substrate under reverse bias reaches from the top surface of the silicon to the buffer layer, which also acts as a depletion layer stop.
Field stop IGBTs, on the other hand, are made starting from a low doped substrate of a first conductivity type without employing epitaxial layers. The field stop zone has considerably lower doping concentration than the buffer layer in a PT-IGBT.
The field stop zone plays a key role in the operational properties of IGBTs. In particular, thickness and concentration of the field stop zone greatly affect the switching and breakdown voltage characteristics of the device.